1. Field of the Invention
This invention relates to gain control circuits and is more particularly directed to a gain control circuit of the voltage-control type, wherein the gain thereof varies as an exponential function of a control voltage applied thereto.
2. Description of the Prior Art
Conventional gain control circuits for electrically controlling the circuit gain use small signal impedance changes to provide corresponding changes of bias voltage or current for non-linear elements, such as semiconductor diodes, bipolar transistors, or field-effect transistors. However, the control characteristics and input/output characteristics of these circuits are far less than is desirable for high fidelity audio equipment, and such circuits are generally unsuitable for use in an audio-signal noise-reduction circuit.
A voltage-control type gain control circuit having a higher performance characteristic has been proposed, for example, in U.S. Pat. No. 3,714,462 to David E. Blackmer. Such circuit takes advantage of the well-known exponential voltage-to-current characteristic of the base-emitter junction of a bipolar transistor. The circuit includes a log-converting transistor and an antilog-converting transistor. Unfortunately, the total static current flowing through these transistors varies greatly as a gain control signal applied thereto is varied, as will be described hereinafter in greater detail.
Because of the significant variations of the static current of such a circuit, a high static current is accompanied by adverse effects such as increased feed-through of the control signal, increased noise (mainly shot noise), and increased current consumption. Conversely, low static current is accompanied with such problems as the generation of crossover distortion. This occurs because of the reduction of the mutual conductance of transistor in the neighborhood of the zero crossing of the input current. Another adverse effect of low static current is the generation of non-linear distortion. This occurs because the operation of the feedback path and output path formed by the PNP and NPN complementarily conductive transistors approach class B operation when the collector-emitter current is low. Still further problems accompanying low static current are the restriction of bandwidth due to the reduction of the cut-off frequency of the transistors, and the instability of the bias circuit with respect to temperature and to source voltage fluctuations.
The actual static current in the prior art voltage-control type gain control circuit is thus selected as a compromise, or trade-off of the aforementioned opposite high- and low-static-current conditions. However, as aforesaid, the variations of the static current with changes of the gain are rather great. Consequently, the available range of selection mentioned above is unavoidably kept narrow. Furthermore, the aforementioned adverse effects cannot be avoided altogether, because of the wide fluctuations in static current that can occur.